Prokaryotic gene expression in mammalian cells involving the purine phosphoribosyltransferase enzymes were investiaed. Micro assays were developed for these enzymes and a bacterial virus carrying the gene for one of the enzymes was used. The number of clones isolated following viral infection was 100-fold greater than those observed in controls. However, electrophoretic analysis of the enzyme produced in these cells was not diagnostic of a bacterial enzyme. Immunological methods are now being developed. The work on galactosemic human cells has been impeded by the lack of good selection systems to isolate cells expressing the galactose genes. Galactose enzyme assays and nucleic acid hybridization techniques were used to demonstrate the discoordinate gene expression of the E. coli galactose operon after induction of a lambda prophage. These studies have led to a realization that controlled gene expression in heterologous systems will require a detailed knowledge of translational and transcriptional control mechanisms.